Selective destruction of leukemia cells has been observed in spontaneous leukemia in AKR and C58 mice, in feline leukemia and dog lymphosarcoma following infusions of normal serum, plasma or whole blood. Heparinized plasma has significantly greater antileukemia activity than serum, and the active factor(s) can be concentrated and partially purified by heparin precipitation or adsorption onto a calcium phosphate gel. We have, in collaboration with Dr. M.W. Mosesson, identified cold insoluble globulin in the heparinized plasma precipitates and preliminary observations indicate that purified CIg appears to contain the same antileukemia activity in both AKR and C58 spontaneous mouse leukemia. This program is directed towards: (1) a study of the antileukemia activity of CIg in the mouse; (2) attempting to determine the relation between CIg and other plasma components in the antileukemia activity; (3) attempting to determine the mechanism of action; and (4) determining whether feline- and canine-purified CIg have activity in feline leukemia and dog lymphosarcoma.